1. Field of the Invention
The present invention relates to a glass blank used for the press molding of an optical element such as lens, and a method for producing the optical element.
2. Related Background Art
Traditionally, means given below have been proposed in order to prevent the fusion between the glass blank and molding mold or the haze occurring in an optical lens after its press molding, when a glass optical element is produced by the press molding.
In Japanese Patent Publication No. 2-1778, a press molding method is disclosed, in which the surface of the inner glass which is the core of a glass blank is covered with a glass having a higher transition temperature than that of the inner glass. Also, in Japanese Patent Publication No. 2-1779, a press molding method is disclosed for a glass blank covered with a silicon dioxide film, in which a glass having a lower transition temperature than that of the silicon dioxide film is used as its glass blank In these two methods proposed, the fusion between the mold and glass blank on molding is prevented by maintaining the surface covering layer in a non-softening state when the press molding is performed.
Nevertheless, in the conventional arts mentioned above, there still exist the problems set forth below.
With the molding methods for a glass blank having an optical glass material as its surface layer, which are specifically disclosed in the above-mentioned two paten publications, the prevention of the fusion between the mold and the glass blank is reliably improved. However, in a case where the optical glass material is processed into a thin film by a vacuum evaporation using an electron gun evaporation source, the glass blank which is the evaporation material is extremely foamed at the time of evaporation, leading to the generation of defects in the film or making it difficult to homogenize the film composition. Such a drawback also occurs in the thin film formation by sputtering with the glass material as its target.
In a case where the silicon dioxide film is used as the surface layer, there is also a drawback in that the generation of a surface crack is conspicuous at the time of molding.